Heater tube arrangements

ABSTRACT

A heater head assembly of the type useful in a hot gas engine is disclosed. The assembly has a plurality of tubes or passages providing a communicating connection for the gas between high temperature working chambers and regenerators. In one embodiment, manifold extensions are provided for each of the working chambers and regenerators and between which banks of heater tubes extend. Each of said manifolds contain at least one blind flow turn-around or reverter structure whereby hot gases traversing between a working chamber and a regenerator must travel the spacing therebetween a minimum of three times. Each of said tube banks are formed by uniting two plys of sheet metal along spaced pairs of parallel lines of brazing, the zone within each pair of brazing lines being expanded to define gas passages for said tube bank and the zone between pairs of brazing lines being perforated to admit cross-flow of a surrounding heating medium.

BACKGROUND OF THE INVENTION

This invention relates to a Stirling engine and particularly to the typeknown as double-acting in which each piston acts as a power piston and adisplacer simultaneously. Heat from an outside heat source such as anexternal flame is added to the engine through a heater head assemblycontaining a working gas. The working gas is expanded to operate apiston within a working chamber; continuous external heating and coolingof the working gas provides for a full cycle engine. The complete cycletakes place in one revolution of the crankshaft as opposed to multiplerevolutions required by conventional piston engines. To make the enginemore practical, regenerators are located between the fixed heating andthe cooling sources; such regenerators store otherwise wasted heatduring the cooling process and permit recovery of the heat during theheating phase. This stored heat is equal to several times the heat addedfrom the outside heat source.

One of the most difficult problems for the Stirling engine is tooptimize heat transfer characteristics through the walls of the heatertubes or pipes, since the output of the engine is dependent thereon. Itis desirable that the heat absorbing surface area of the heater headassembly or tube complex should be as great as possible and that theheat flux be as great as possible. However, the volume of the gas in thetubes or pipes must be as small as possible as this volume is a "dead"volume in the working cycle; similarly it is desired that the resistanceagainst the flow of the working gas through the heater tubes should beas low as possible.

Heretofore, prior art constructions have attempted to meet suchconflicting goals by utilizing tubes of a very small internal bore andusing such tubes in great numbers to permit a large surface area to comein contact with the working gas passing within. The tubes were arrangedto cause the working gas to flow on a singular path from the workingchamber to or from the regenerator. In most instances, such tubes wereexposed to one single pass of the surrounding heating medium and incertain instances, as described more fully in the detailed description,a partial double pass was provided. With these constructions, thetemperature of the exhaust gases, having passed through and against suchheater tubes, was at an extremely high temperature range indicating thatthe heat content thereof was not transferred as efficiently as possibleto the working gas within the tubes.

To meet this problem, regenerative wheels have been utilized so thatheat could be collected from the exhaust gases and returned to theincoming air to be combined with fuel for combustion; thus the exhaustheat content was in part returned to the cycle. Such regenerative wheelspresent many attendant problems including unneccessary cost.

In another attempt to meet this problem, the heat tubes were arrangednot only to project linearly from the working chamber but wereadditionally bent to have a U-shape and/or connected to a ring-shapedmanifold located above the working chamber and regenerator. Thesearrangements have not allowed an increase of the number of pipes due tospace. Accordingly, it was necessary to increase the length of the pipesand thus also to increase the resistance against the gas flow therein.Fins or heat conducting surfaces were added to the lengthened portionsof the pipes to improve heat transfer; such fins have proved to becostly, fragile and difficult to manufacture. It has become evident thatthis mannner of increasing heat absorbing capacity is limited and incertain respects undesirable.

SUMMARY OF THE INVENTION

The primary object of this invention is to provide a hot gas engine ofthe type which utilizes a heater head assembly for imparting heat to aclosed working gas system, the heater head assembly being characterizedby improved heat transfer performance permitting elimination ofauxiliary heat restorative devices while insuring that exhaust gas exittemperatures are 600°F or below.

Another object is to provide an improved heater head assembly for a hotgas engine by deploying reverter elements in each of the headersconnected to a working chamber or regenerator, the distance between eachassociated working chamber and regenerator a great number and thus moreeffectively exposed to cross-flow of the surrounding heated medium.

Yet another object of this invention is to provide an economical methodand construction for a heater tube assembly which facilitates automatedmanufacture of a unitized heater head assembly having controlled finesizing of passages for hot gas flow.

Specific features pursuant to the above objects comprise the use ofstamped heater tube banks formed from plys of sheet metal which areunited together at spaced pairs of brazed lines, flow passages beingdefined within each pair of lines and the zone between pairs of lines isperferred to permit cross-flow of the surrounding heated medium.Preferably, a series of at least three banks extend between manifolds orheaders connected to each of the regenerators and working chambers, eachmanifold having a blind flow turn-around or reverter element integrallycast as an internal dividing wall.

SUMMARY OF THE DRAWINGS

FIG. 1 is a schematic elevational view of a one prior art Stirlingengine of the double-acting piston type having upright manifoldsprovided with horizontally arranged heater tubes extending therebetween;

FIG. 2 is an enlarged schematic sectional view of the heater headassembly of FIG. 1;

FIG. 3 is an enlarged view similar to a portion of FIG. 2 but embodyingthe principles of this invention;

FIG. 4 is a view taken substantially along line 4--4 of FIG. 3;

FIG. 5 is a side elevational view of a tube bank of this invention inone stage of manufacture;

FIG. 6 is an end view of the structure of FIG. 5 illustrating a step inthe fabrication of the tube bank;

FIG. 7 is a schematic perspective view of another type of prior artStirling engine, the regenerators and working chambers are arrangedabout a cylindrical cluster; and

FIG. 8 is a diagramatic veiw taken along a plane passing horizontallythrough the heater tube assembly of FIG. 7, but embodying the principlesof the present invention and thereby representing an alternativeembodiment.

DETAILED DESCRIPTION

Exchanging the heating and cooling sources is a cumbersome process.Therefore common Stirling engines replace the alternating use of hot andcold sources by addition of a mechanism called a displacer piston whichserves to move the gas between a stationary hot chamber and a stationarycold chamber. The displacer piston mechanism allows the heating sourceto be stationary at one end of the cylinder and the cooling source to bestationary at the other end. When the displacer piston moves upward, thehot working gas from the upper portion of the cylinder is first movedthrough heater tubes and then flows through the cooling coil where it iscooled until most of the working gas is in the cold section below thedisplacer piston. Because the gas is cool, its pressure is low. Movingthe piston downward forces the working gas back through the coolingcoils and into the heater tubes where it is heated and forced into thehot section above the displacer piston. Since the gas is now hot, itspressure is high. There are no valves in the flow path, so that when theupper chamber is at high pressure, the lower chamber is also at highpressure.

The embodiments disclosed herein to illustrate the invention are hot-gasengines of the double-acting type. In this type, each piston serves asboth a power piston and as a displacer piston for an adjacent workingcylinder, thus the name double-acting. This type makes it possible toconstruct an engine having four separate interconnected cylinders andcontrol the motion of the pistons by device which phases them at 90°intervals.

To point out the inventive contribution of FIG. 3, the prior artconstruction of FIG. 1 shall be discussed first. The latter is aconstructive popularly promoted by United Stirling of Sweden and has aplurality of working chambers alternating with a plurality ofregenerators, all arranged about a circle. The working chambers may havetheir longitudinal extent canted to form a V-4 or V-8 relationship, suchas in a piston engine.

In particularity, engine 9 employs a cylinder wall 10 together with apiston 11 to define a high temperature working chamber 12. Asubstantially upright manifold 13 extends from an opening 8 in the upperportion of wall 10. A plurality of arcuately shaped heater pipes ortubes 14 extend generally horizontally parallel to each other to providea closed communication between vertically aligned openings 7 in manifold13 and vertically aligned openings 6 in a similar manifold 15 secured toand communicating with an opening 5 in the top of regenerator 16; theregenerator is in turn mounted on a cooler 17. The cooler 17 is securedto the portion 18 of the engine. Another regenerator 19 and anothercooler 20 are shown in sectional view. The working gas from the heaterhead assembly will pass through a regenerator and a cooler through aduct 21 to a low temperature working chamber 22 located under the piston11. Each cooler comprises parallel pipes 23 extending vertically andsurrounded by a flow of cooling agent. Each cooler is rigidly connectedat its bottom to the engine portion but is flexible to allow sidewaysmovement of the top of the cooler. Sideways movement will occur whenthere are changes in the temperature of the pipes following starting andstopping of the engine.

Working gas in the closed system between each working chamber andregenerator, travels a single pass when traversing the distanceseparating the associated manifolds (i.e. 13 and 15). This is bestillustrated by reference to FIG. 2. The tubes 14 can be seen to bearranged in a double layer 14a and 14b; each layer is shown with 7 tubesaligned along an upright direction and therefore a total of 14 tubesinterconnect each pair of associated manifolds. Four working chambersand four regenerators are utilized and disposed in a ring about theinterior combustion space having a heat source 25. The upright manifolds(24-29 and 13, 15) emanating from these working chambers andregenerators, would be arranged as in FIG. 2 with double-layer tubingsystem therebetween.

The total heat absorbing surface area of the heater head assembly ofthis prior art construction is useful because the manifolds are madetaller than other prior art concepts; the tubing is increased in numberand at higher elevations.

Unfortunately, the heated medium (exhaust gases from the combustion at25) flows past each tube only once and any heat transfer must take placequickly during this momentary interchange; considerable residual heat isretained in the exhaust gas after passing through the header heatassembly which results in an inefficient engine and other relatedproblems.

In accordance with the present invention and as shown in FIG. 3, theheater tube assembly preferably has a minimum of three tube banks,designated 30, 31 and 32, extending between manifold or headers 33 and34. Manifold 31 may be connected to a working chamber and manifold 32may be connected to a regenerator. Each manifold is subdivided by anintegrally cast wall 35 and 36 respectively; the wall in conjunctionwith a blind closure member (not shown) at either end of the manifold,defines a flow turn-around means or reverter element which functions toprevent flow (see arrows) from communicating with a working chamber orregenerator to which the specific manifold is directly connected. Theinterior of each manifold is accordingly divided into two portions, afirst portion 37 communicating directly with a working chamber orregenerator and portion 38 has flow communication only with tube banks30 and 31. Flow emanating from one manifold and traveling to the othermanifold is returned by the turn-around means once at each manifold. Toprovide this result, the ends of each tube bank must be sequence asfollows: tube bank 30 has the entrance openings 39 for each of thepassages 40 connected to complimentary openings 41 entering upon portion37 of the manifold 34 which communicates with the regenerator. Theworking medium, such as hydrogen, is caused to flow through the passages40 of the tube bank 30 and exit from openings 42 into portion 38 ofmanifold 33, Flow is reverted to bank 31 (see arrows) by enteringentrance openings 43; portion 38 is closed except for said lattercommunication. The exit openings 44 for tube bank 31 enter upon theportion 38 of manifold 34. The reverter or flow turn-around means ofportion 38 prevents flow from being in communication with theregenerator; accordingly, having traveled twice the distance 45separating the two manifolds, is caused to return a third time byentering upon the entrance openings 46 of tube bank 32, passingtherealong to enter the portion 37 of manifold 33. Portion 37 is incommunication with a working chamber.

The above is a description of flow for one momentary cycle of theengine, it being understood the sequence of flow is reversed repeatedlyand therefore the entrance and exit terminology must be exchanged. Thecombusted gases serve as a surrounding heating medium which flowscrossways through openings 47 in the margins 48 of the tube banksseparating the passages 40. As best shown in FIG. 4, the cross-flowencounters the closed flow of the working medium a minimum of threetimes, thereby increasing the heat exchange capabilities.

Now turning to FIGS. 5 and 6, the heater tube banks are illustrated withrespect to the method of manufacture. In the past, heat exchangedevices, of the type generally shown here, have been made by uniting twoflat metallic sheets which are pressure welded together at separatedpatterns; the unbonded areas between said patterns are distended byinflation to produce tubular passages. Such distention cannot define thevery fine, almost hypodermic needle size passages which are required forthe Stirling type heater head assemby. In this invention, the separateplys 50 and 51 of sheet metal, utilized to form the tube bank arestamped with grooves 52 having a hemi-spherical section; the grooves arearranged parallel and separated by a distance 53 equal to or less thanthe diameter of a completed passage 54. During the stamping operation,perforations or elongated openings 47 are defined in the contiguousmargin zone 55 between the grooves 52; openings 47 permit the combustiongases to pass therethrough. The perforations are arranged on adjacentbanks so that a non-direct flow of heating medium must pass therearound.Continuous lines of uniting are provided at 56 adjacent the groove orpassage. The plys are mated together so that the contiguous margins at56 are in solid contact; continuous welding rollers 58 are employed toprovide a welded seam along such uniting lines.

A preferred method would comprise the following steps:

a. prepare cast metallic headers for connection to each working chamberand regenerator, each header having a cylindrical tube with an integralwall separating the interior of said tube into first and second portionsand being supported in an upright orientation.

b. prepare a tube bank, for connecting with two of said headers, byuniting two plys of highly heat conductive sheet metal along spacedpairs of lines. Between, but not within every pair of lines, a tubepassage is preformed in each ply by a groove having a hemi-sphericalcross-section; when the plys are mated, the complimentary grooves definea cylindrical passage. Within each pair of lines, elongated perforationsare punched to provide freedom for a surrounding medium to flowcrossways through said bank, and

c. providing rows of openings along the length of each header, each rowto receive the ends of one bank of expanded passages for conducting gasthereto, two rows of said openings being in communication with the firstportion of said header and the other row being in communication with thesecond portion of said header, said first portion being sealed toprevent communication with either said working chamber or regeneratorand the tube ends being sealed to said headers while disposed in saidopenings.

FIG. 7 illustrates another type of double-acting piston Stirling engineaccording to the prior art. This construction is properly known as theswash plate design. Four working chambers or cylinders 60 and eightregenerators 61 are wrapped about a cylindrical outline 63. A singlecentralized head source is utilized (burner unit 62); each cylinder 60is connected to two regenerators 61 by a tube labyrinth 64. Thelabyrinth is comprised of discrete and independent tubes containing theworking gas, preferably hydrogen. Cooler tubes (not shown) are connectedalso to the respective cylinders and regenerators; the cooler tubes aregenerally located between and slightly outboard of the cylinders.Pistons in each cylinder 60 have rows extending axially therefrom tocontact a swash plate (not swash shown); four torque impulses phased at90° intervals are imparted per revolution of the plate, similar to aneight cylinder internal combustion engine but with impulses of a smallermagnitude.

Each tube 64 of the labyrintth has a first portion 64a which isuprightly parallel to the axis of cylinder 63; each tube is providedwith a hair-pin turn at 64b and a portion 64c is arcuately spiralled tointerconnect with a regenerator. combusted gases from within thecylinder 22 and about burner unit 62 pass radially outwardly, no morethen twice, past the working flow within the tubes 64. More heat isextracted from tube portion 64a than 64c; accordingly it is common toemploy fins and auxilliary heat exchange surfaces along portion 64c topromote greater heat transfer. This, however, introduces undesirablemanufacturing problems.

The swash-plate design of FIG. 7 can be redefined according to theprinciples of this invention to provide the construction shown in FIG.8. Working chambers or cylinders 71, 72, 73 and 74 and regenerators75-82 are again arranged about a cylinder with source 83 located at theaxis. Headers 84 having reverter elements 85 extend uprightly from eachworking chamber and a header 86 having a reverter element 87 extenduprightly from a transition section 88 interconnecting two regenerators.The function of a multiple pass of the surrounding heating medium acrossthe tube layers 90, 91, 92 is similar to that in the preferredembodiment of FIGS. 3 and 4.

I claim as my invention:
 1. A heater head assembly of the type useful ina hot gas engine of the Stirling type wherein a labyrinth of heatconductive tubes provide a closed connection for trapped gas at highpressure to reversibly communicate between a high temperature workingchamber and a thermal regenerator, said labyrinth of heat conductivetubes selectively receiving heat from a central gaseous heat sourcewithin said labyrinth of tubes and promoting absorption of heat fromsaid source to the trapped gas therein which is at a relatively lowertemperature equal to or greater than 600°F, the assembly comprising:a. afirst header connected to at least one high temperature working chamber,b. a second header connected to at least one thermal regenerator, c. acontinuous passage labyrinth having inner and outer cylindrical walls,an entrance communicating with one of said first and second headers andan exit communicating with the other of said first and second headers,said labyrinth being characterized by at least one element associatedwith each header to revert gas communication after having traveledsubstantially the distance separating said first and second headers,whereby flow of said gaseous heat source through said labyrinth willconduct heat to an increased inner wall area for improving heat transferto the high pressure trapped gas therein.
 2. The assembly as in claim 1,in which said reverter elements are integrally formed as part of eachheader.
 3. The assembly as in claim 1, in which the headers are definedas cylindrical tubes having an integral cast wall dividing the interiorthereof into two portions, one portion being out of direct communicationwith either said working chamber or regenerator but in communicationwith more than one portion of said labyrinth to function as a reverter.4. A heater head assembly of the type useful in a hot gas engine of theStirling type wherein a labyrinth of heat-exchange tubes provide aclosed connection for high pressure trapped gas to communicate between ahigh temperature working chamber and a thermal regenerator whilereceiving heat from a surrounding heated gas medium, the assemblycomprising:a. a first header manifold connected to at least one hightemperature working chamber, b. a second header manifold connected to atleast one thermal regenerator, and c. a tube bank extending between saidfirst manifold and said second manifold, each tube bank havingindividual tubes with inner and outer cylindrical walls extendingsubstantially between said first and second manifolds and being spacedto permit flow of said heated gas medium therebetween with flow oftrapped gas therein which is at a relatively lower temperature equal toor greater than 600°F, each manifold being divided by interior wallmeans to provide at least one flow reversion whereby gas beingcommunicated through one tube is returned once through another tube andreturned again through still another tube before being allowed to enterthe chamber or regenerator with which the manifold is associated,whereby flow of said heated gas medium through said tube bank willconduct heat to an increased inner wall area for improving heat transferto the high pressure trapped gas therein to enhance the heat transfercharacteristics of said assembly.
 5. The assembly of claim 4, whereinsaid manifolds each extend generally vertically upward from a respectiveregenerator or working chamber, and said tube banks each are arrangedwith individual tubes thereof extending generally horizontally betweensaid manifolds, and each of said manifolds is provided with an interiorwall effective to provide a flow turn-around for one selected group oftubes entering said manifold.
 6. The assembly as in claim 4, in whichsaid working chambers and regenerators are clustered together to definea cylinder, said manifolds extend upwardly from each of said workingchambers and regenerators and are aligned generally parallel withrespect to the axis of each of said working chambers or regenerators,and said tube banks extend along an arcuate segment of the periphery ofsaid cylindrical cluster with the tubes in each tube bank beinggenerally arranged in layers parallel to the axis of said cylindersregenerator.
 7. The assembly as in claim 1, in which a plurality offirst and second headers are employed and are arranged in a clusterabout the periphery of a cylinder with a source of heat being disposedat the axis of the cylinder, the first headers being spaced slightlyradially inwardly of the second headers, each labyrinth having anaduncous shape effective to interconnect one first header with onesecond header and have a substantial portion thereof aligned with theperiphery of said cylinder.
 8. A heater head assembly of the type usefulin a hot gas engine of the Stirling type wherein a labyrinth ofheat-exchange tubes provide a closed connection for high pressuretrapped gas to communicate between a high temperature working chamberand a thermal regenerator while receiving heat from a surrounding heatedgas medium, the assembly comprising:a. a first header manifold connectedto at least one high temperature working chamber, b. a second headermanifold connected to at least one thermal regenerator, and c. a tubebank extending between said first manifold and said second manifold,each tube bank being formed of sheet metal plys joined together atseparated margins with gas passages formed between said margins andextending substantially between said first and second manifolds, saidmargins having apertures therein for permitting flow therethrough ofsaid heated gas medium, the flow of trapped gases in said passages beingat a lower temperature relative to said heated medium but equal to orgreater than 600°F, each manifold being divided by interior wall meansto provide at least one flow reversion whereby gas being communicatedthrough one tube is returned once through another tube and returnedagain through still another tube before being allowed to enter thechamber or regenerator with which the manifold is associated, wherebyflow of said heated gas medium through said tube bank will conduct heatto an increased inner wall area for improving heat transfer to the highpressure trapped gas therein to enhance the heat transfercharacteristics of said assembly.